What is the legacy of Manifest Destiny? In other words, what "gifts" did it leave Americans—good and bad?

Manifest Destiny was an idea that sprang up in the early to mid-nineteenth century. It asserted that it was obvious that the destiny (fate or God-given imperative) of the United States was to control the entire central portion of the North American continent from the Atlantic to the Pacific oceans.
The positive gift (at least to white Americans at the time) of this ideology was that it led to the United States coming to control this vast territory. The obvious negative "gift" was that the idea of Manifest Destiny was tied to notions of white supremacy. White Americans felt that they had the right to rid the land that they felt was manifestly or obviously "theirs" of its pre-existing occupants, the many Native tribes with robust cultures that already populated this territory. Instead of negotiating with the Native Americans to coexist peacefully, white Americans committed calculated and careless genocide against the Native tribes.